The present invention relates to space and communications satellites, and more particularly, to a coordinated system for providing satellite communications using multiple satellites.
Wired terrestrial systems offer communications at high data rates, but only while the user is sitting behind a computer. As soon as the user goes to a conference room, walks outside an office building, gets into a car, or drives to a park, the connection is lost. Mobility, however, can be supported in one of two ways, namely terrestrial-based wireless networks or satellite-based communications systems.
Terrestrial-based wireless networks provide voice or data communications between a mobile user and a fixed user or to other mobile users, as well as communications for modem-equipped computers and other similar devices such as mobile facsimile machines. Existing wireless networks have not been optimized for a mix of voice, data, and video, however, despite the trend towards multimedia traffic. Several wireless and wired standards, such as asynchronous transfer mode (ATM), are being designed to optimize multimedia traffic. Wireless wide area networks (WANs) typically carry voice, whereas wireless local area networks (LANs) typically carry data. Most wireless WAN traffic operates at under 19.2 kbps. Wireless LANs that support data rates up to 10 Mbps have begun to appear, but they are limited in range to tens of meters.
To provide wireless service, satellite-based communications systems have been proposed which would provide world-wide coverage. These proposed systems typically include a constellation of satellites in one orbit only, such as geostationary earth orbit (GEO) only or non-geostationary orbits (NGSO). Communications satellites in geostationary orbit provide coverage in predetermined areas on the earth from the equator. Coverage is typically excluded from the oceans so that satellite capacity is not wasted on non-populated areas. Communications satellites in geostationary orbit, however, provide limited coverage at higher or lower latitudes than the Equator.
Communications satellites in non-geostationary orbit, such as medium earth orbit (MEO) or low earth orbit (LEO), travel relative to the Earth""s rotation and typically provide high elevation angle coverage at the higher and lower latitudes, and since they are closer to earth, propagation time delays are minimized. Because of the unavailability of stationary positions it is desirable to deploy NGSO satellites.
In one known implementation of a NGSO satellite system, several disadvantages are apparent. In the known system, each satellite is deployed in its position individually. One drawback to individual launches is the high cost associated with each launch. Another drawback is that the system is not easily adaptable to increasing demand. Individual launches must be used to provide increased coverage.
Data rates up to 19.2 kbps, as available from wireless WANs, will not meet future data rate needs of consumers. For example, many computer users are upgrading their wired modems to 56.6 kbps whenever possible. Such users desire a fast response from their modems even while they are away from their desks. In addition, the nature of the information being transferred is changing from short, text-based electronic mail messages to communications with embedded video clips. Such media-rich messages consume high bandwidth and communications resources, thus requiring high data rates to allow them to be transmitted and received within a reasonable period of time.
Furthermore, a tremendous growth in Internet traffic has caused a strain on the capacity of telephony networks. Network shortcomings include network outages, insufficient access bandwidth, and insufficient internode bandwidth. Currently, providers need to make significant investments, as well as experience installation delays, to upgrade network infrastructure, yet they cannot pass the costs on to the end users.
Corporate LANs/WANs also generate an insatiable demand for higher bandwidth. The demand for bandwidth goes up as more and more users are connected. The users, in turn, demand more services and improved network speed. Personal computers are being used to process not only text, but graphics and video as well, all on networks that are increasingly global. Widespread implementation of corporate intranets and extranets further drive the move to increased bandwidth applications. High-speed networking is also driven by the growth of video distribution, client/server technology, decentralized systems, increased processing power and developments in storage capacity.
Fixed service demand such as satellite news broadcast, distance learning, and military functions are continually increasing. It would be desirable to provide a system capable of meeting demand of such uses.
Thus, there exists a need for a satellite communications system that provides communications to mobile users as well as fixed service users. There also exists a need for a satellite communications system that provides global communications service while maximizing the useful capacity of the satellites, reducing the perceived time delay, and maximizing the minimum elevation angle across latitudes.
The present invention provides a satellite communications system which provides global network services to fixed and mobile users. The system utilizes a first deployment of a plurality of satellites deployed in a medium earth orbit (MEO) and a few subsequent deployments of a plurality of satellites deployed in the same medium earth orbit (MEO) or other orbits. A ground terminal is provided for communicating with the first and the later deployments.
In one aspect of the invention, the satellites may be deployed in at 15000 km. One advantage of using 15000 km is that the satellites avoid interference with the Van Allen radiation belts. Another advantage is that polar orbiting satellites need not be deployed.
One advantage of the invention is that a one dimensional tracking ground antenna may be employed. A one dimensional tracking antenna is less expensive than two-dimensional antennas.
Another advantage of the invention is that the system is extremely adaptable in a business sense. That is, the system can be deployed in a first configuration. Then, as the needs of the users of the system increase, further satellites may be deployed. The first deployment may be spaced to easily accommodate the second deployment so that the second deployment may be accomplished in a single launch.
Another advantage of the invention is that the constellation of the present invention promotes frequency reuse. That is, because the MEO satellites of the present invention are not in a direct line with GSO satellites, the frequencies of GSO satellites may be reused in the present constellation.
Yet another advantage of the present invention is that some time will lapse between the initial deployment and later deployments. Thus, the later deployments may take advantage of the newest technology, which is important in the rapidly changing satellite technology industry.